This invention relates to a unique carrier, axle differential, and inter-axle differential assembly configuration for a tandem drive axle.
Tandem drive axle assemblies include a forward drive axle and a rear drive axle interconnected by a driveshaft. A single driving input is operably coupled to the forward drive axle, which includes an inter-axle differential (IAD). The IAD splits the driving force from the input between the forward and rear drive axles. A thru-shaft interconnects the IAD to the driveshaft that provides input to the rear drive axle.
The forward and rear drive axles each include a carrier with a differential gear assembly to prevent wheel skid during turning maneuvers. When a vehicle travels along a straight-line path, both sets of wheels on a drive axle will turn at basically the same speed. During a turning maneuver, however, the wheels on the outside of the turn must travel a greater distance than the wheels on the inside of the turn, which means that the wheels on the outside of the turn must rotate at a faster speed than the wheels on the inside of the turn. A differential gear assembly is required to allow for this difference in wheel speed.
Traditionally, the forward drive axle carrier includes helical gear set that transfers the driving force from the input at the IAD to a ring and pinion gear set that is operably coupled to the differential assembly. The differential assembly includes a first differential case half, a second differential case half, and a differential gear set. The ring gear is bolted to one of the case halves to define a first bolted joint and the first and second case halves are bolted together to define a second bolted joint.
The helical gear configuration is also required to permit the thru-shaft to pass the differential case assembly. This configuration severely limits the overall size of the differential because sufficient clearance is required to allow the thru-shaft to operate. The thru-shaft is mounted within the forward axle housing by a pair of bearings supported by a separate cage member that is bolted to the housing.
This traditional differential case and thru-shaft configuration is expensive to manufacture and difficult to assemble. Also, with the increased demand by users to provide more robust designs within the same packaging space, these traditional configurations do not provide room to make critical components more robust within the existing package. Further, the IAD and helical gear configuration often require a separate pumping mechanism to force feed oil through the assembly. This additional pumping mechanism increases cost and adds weight to the assembly, which is undesirable.
Accordingly, it is desirable to provide an improved forward drive carrier with a differential assembly that includes a more robust component configuration within the same package. Further, it is desirable to provide a simplified carrier and thru-shaft configuration that reduces the overall number of required components and is less expensive to manufacture, as well as overcoming the other deficiencies in the art outlined above.
A tandem axle set includes a forward drive axle and a rear drive axle that are coupled together with a connecting driveshaft. The forward drive axle includes a forward carrier assembly coupled to a vehicle input and which drives a forward pair of axle shafts. An inter-axle differential (IAD) in the forward carrier assembly splits driving force between the forward and rear drive axles. A thru-shaft is coupled to the IAD at one end and to an output at the connecting driveshaft at an opposite end. The connecting driveshaft is coupled to drive a rear carrier assembly that drives a rear pair of axle shafts.
The forward carrier assembly includes a forward drive gear assembly that is operably coupled to the forward pair of axle shafts. The forward drive gear assembly includes a pinion gear, a ring gear, and a forward differential assembly. The differential assembly includes first and second differential case halves attachable at a case interface to define a case split line. The differential assembly also includes a differential gear assembly supported by the first and second differential case halves with the gear assembly being operably coupled to drive the forward pair of axle shafts. The ring gear is mounted to the differential case halves. The IAD provides driving power to the pinion gear that meshes with the ring gear to drive the axle shafts via the differential gear assembly.
The IAD includes differential spider, a plurality of spider gears supported on the differential spider, an inner side gear in meshing engagement with the spider gears, and an outer side gear in meshing engagement with the spider gears. The. differential spider, spider gears, and inner and outer side gears are substantially enclosed within an IAD housing. The housing is rotatably supported on an IAD bearing assembly. The pinion gear is mounted for rotation with the inner side gear to provide driving input to the forward drive axle. The thru-shaft is splined for rotation with the outer side gear to provide driving input to the rear drive axle.
In the preferred embodiment, the pinion gear. includes a first piece defining a pinion gear head and a second piece defining a hollow pinion support shaft that extends into the IAD. The thru-shaft extends through the hollow pinion support shaft such that the pinion gear and thru-shaft rotate about a common axis. The pinion support shaft has an inner end that supports the first piece and an outer end that extends into the inter-axle differential assembly for applying a thrust load to the inter-axle differential bearing assembly to permit reverse load sharing.
In one disclosed embodiment, the pinion gear is supported by a pair of bearings including an inner bearing and outer bearing positioned on opposing sides of the pinion gear head. The outer bearing is supported on the first piece and the inner bearing is supported on the second piece. Preferably, the inner and outer bearings are tapered roller bearings.
The first piece of the pinion gear includes a hollow sleeve portion that extends outwardly from the pinion gear head in a direction toward the IAD. The second piece, defining the pinion support shaft, extends through the hollow sleeve portion. The inner end of the pinion support shaft extends beyond the pinion gear head to support the inner bearing. The pinion support shaft includes a center flange portion that abuts against a distal end of the hollow sleeve portion. Preferably, the pinion support shaft is fixed for rotation with the inner side gear at a center position between the inner and outer ends and adjacent to the center flange portion.
Preferably, the outer end of the pinion support shaft extends into the IAD to abut against the outer side gear. Thus, the thrust load is applied to the IAD bearing assembly via the outer side gear. Further, the thru-shaft extends beyond the pinion support shaft and is fixed for rotation with the outer side gear.